This invention relates to improvements in lift trucks for handling unpalletized loads. The invention is applicable both to high-lift and low-lift trucks, and in particular features a load-carrying platform which is rotatable about a generally vertical axis of rotation.
A common problem in the handling of unpalletized loads (i.e. normally those having heavy paper or cardboard slip sheets beneath them as opposed to rigid pallets) is the transfer of such loads to or from conventional rigid pallets. It has long been the practice in the materials handling industry, for maximum space utilization and handling efficiency, to arrange loads in units of generally elongate, rectangular proportion, and to support such loads upon pallets of generally corresponding elongate, rectangular proportion. A common type of pallet utilized, for example, has the shape of a rectangle with longitudinal sides measuring 48 inches and ends measuring 40 inches, with spaced parallel slats or boards providing the load-bearing surface and extending in a direction parallel with the ends of the pallet, i.e. transverse to the longitudinal dimension of the pallet. Such rigid pallets are normally utilized to support loads in warehouses and other storage facilities where available space is not severely limited and the additional space occupied by each pallet is more than compensated for by the facility which the pallets provide for manipulating the loads.
However, when a load is to be moved from a warehouse into a highway truck or trailer or other vehicle having limited cargo space, the space occupied by the rigid pallets becomes a liability and the practice has therefore developed to depalletize the loads and place them in the vehicle on slip sheets. Since the interior width of highway trucks or trailers available for placement of the loads is generally less than eight feet, it is necessary that some of the elongate loads be oriented transversely and others oriented longitudinally of the vehicle for maximum utilization of space in an arrangement known as "pinwheeling" of the loads, i.e. with the loads oriented perpendicularly to one another. This presents a problem both in the loading and unloading of the vehicle, particularly where such loading or unloading is accompanied by depalletizing and palletizing respectively of the loads. The problem arises from the fact that, in the depalletizing or palletizing of a load, it is necessary for a lift truck to move the load in a direction parallel to the direction of the load-bearing slats of the pallet (i.e. transverse to the longitudinal dimension of the load and pallet) so that the load, during depalletizing or palletizing, may slide with respect to the pallet along the longitudinal direction of the slats. Attempting to slide the load in a direction transverse to the slats results in impediments to the desired sliding motion, caused by the edge of the load catching in the longitudinal spaces between adjacent slats. Thus the lift truck must always, at some phase in the operation, orient the elongate load so as to be able to engage or deposit the load by movement (usually in the direction of travel of the truck) which is transverse to the longitudinal dimension of the load. This presents no problem where the load's orientation in the vehicle, for example, is transverse to the direction of travel of the lift truck. However, in the case of "pinwheeling" as described above, only 50% of the loads will be so oriented in the vehicle. For the remaining 50% of the loads the lift truck must at some point in the operation deposit the load in one orientation with respect to the lift truck and pick it up again in an orientation perpendicular to the original orientation. This extra step results in a substantial loss of time in depalletizing and palletizing operations and has a significant adverse effect on the overall efficiency and speed of such operations.
In the past, certain depalletizing devices of the general type exemplified by Sinclair U.S. Pat. No. 3,645,409 have been developed to aid in the transfer of loads from pallets. Some such depalletizing devices permit the lift truck to engage the depalletized load from either the side or the end of the load; however the decision must be made at the time of engaging the load, rather than later at the time of depositing the load in the vehicle, which orientation is desired. Moreover such devices are quite costly and thus their use is not particularly widespread. Furthermore such devices do not solve the aforementioned problems connected with the reverse operation of unloading and subsequent palletizing of loads.
Of course a great many lift truck load-handling attachments are known which feature load-carrying forks rotatable about a generally vertical axis. Exemplary of these are Ulinski U.S. Pat. No. 2,709,017, Hansen U.S. Pat. No. 3,672,526 and Smith, Jr. U.S. Pat. No. 3,701,446. However such prior art rotatable fork assemblies, besides being suitable generally for handling only palletized loads, are capable of rotating only between a side-engaging or depositing position and a front-engaging or depositing position, with the load in both cases having the same rotational orientation relative to the direction in which the truck moves the load during the engagement or deposit thereof. Thus, for example, if an elongate load is oriented longitudinally on the forks when the forks extend forwardly, rotating the forks of the prior art devices 90.degree. does not permit the load to be slid onto a pallet with the load's longitudinal dimension transverse to the direction of sliding and to the slats. Rather, the rotation of any of the prior art devices to such a position that their load-handling forks extend transversely to the direction of travel of the lift truck (i.e. in their side-loading orientations), merely forces the load to be slid in a sideways direction, but still in a direction parallel with the longitudinal dimension of the load. The load, in the transverse orientation of the forks, cannot be slid in a forward direction relative to the truck, i.e. transverse to the forks and to the longitudinal dimension of the load, as would be necessary for proper palletizing because the load cannot slide transversely to the forks. Rather the load must always slide in the same direction as the direction of extension of the forks.
Accordingly it would be impossible to use such devices to extract an unpalletized load longitudinally from a vehicle forwardly of the lift truck and then deposit the load in proper orientation on a pallet without an intervening deposit and reengagement of the load. Likewise it would be impossible to depalletize a load and then deposit the load longitudinally in a vehicle forwardly of the lift truck without such intervening steps. Yet it is exactly these capabilities which are required to improve the speed and efficiency of the above-described load-handling operations.